1. Field of the Invention
The present invention relates in general to drive systems for four wheel drive vehicles and more particularly to a four wheel drive unit automatically engaged and disengaged to provide four wheel drive and two wheel drive selectively in response to variations of vehicle travelling conditions.
2. Description of the Prior Art
A prior art four wheel drive unit of the above mentioned kind, as for example disclosed in the Japanese Provisional Patent Publication No. 58-30835, consists of a transfer clutch adapted to be operated by line pressure of a hydraulic control unit of an automatic transmission in such a manner as to provide four wheel drive and two wheel drive selectively in response to variations of vehicle travelling conditions, i.e., in such a manner as to provide four wheel drive upon low-speed high-load travelling conditions and two wheel drive upon high-speed low-load travelling conditions of the vehicle.
The prior art four wheel drive unit, however, is not adapted to actively respond to whether the vehicle is travelling a corner. Due to this, even upon turning on a dry, smoothly surfaced road where two wheel drive is desirable, the drive unit may possibly be engaged to provide four wheel drive, causing problems of a tight corner braking phenomenum and an inability of controlling the behaviour of the vehicle through control of an accelerator pedal.